1. Field of the Invention
The present invention relates to a multi-beam optical scanning apparatus and an image forming apparatus using the same. More particularly, the present invention relates to an optical scanning device suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which has, for example, an electrophotographic process and employs a structure in which a plurality of light beams emitted from a light source means having a plurality of light emission regions are deflected by a polygon mirror serving as an optical deflector and then a surface to be scanned is scanned with the light beams through an imaging optical system having an fθ characteristic to record image information.
2. Related Background Art
Up to now, in an optical scanning device, a light flux (light beam) which is optically modulated according to an image signal and emitted from a light source means is periodically deflected by an optical deflector composed of, for example, a polygon mirror. The light beam is condensed in a spot shape on the surface of a photosensitive recording medium by an imaging optical system having an fθ characteristic. The surface of a photosensitive recording medium is scanned with the light beam to record an image.
In recent years, with increase in speed and downsizing in an image forming apparatus as a laser beam printer, a digital copying machine, or a multi-function printer, it is desired to further increase a scanning speed of a scanning optical system used as an optical system and to make the scanning optical system more compact. In order to increase the scanning speed, for example, a multi-beam optical scanning apparatus is used.
In the multi-beam optical scanning apparatus, a plurality of lines on a surface to be scanned are simultaneously scanned with a plurality of light beams emitted from a light source means having a plurality of light emitting regions (light emitting points), so that image information is recorded at high speed. Referring to FIG. 9, a multi-beam semiconductor laser will be described as an example of the light source means used for the multi-beam optical scanning apparatus.
FIG. 9 is a principal part schematic view showing a multi-beam semiconductor laser having two light emitting regions.
As shown in FIG. 9, a multi-beam semiconductor laser 91 has two light emitting regions 94a and 94b in an active layer 93. Deflection directions of divergent light beams emitted from the two light emitting regions 94a and 94b coincide with each other. Sectional shapes 95 of the divergent light beams are substantially identical to each other.
When two lines on a surface to be scanned are simultaneously scanned with the two light beams using the multi-beam semiconductor laser 91, an interval between the two lines can be adjusted by changing an interval between the two light emitting regions 94a and 94b of the multi-beam semiconductor laser 91 in a sub scanning direction. The interval between the two light emitting regions 94a and 94b is determined in advance. Therefore, the interval in the sub scanning direction is adjusted by rotating the multi-beam semiconductor laser 91 about an intermediate point between the two light emitting regions 94a and 94b as the rotational center.
An overfilled optical system (hereinafter referred to as “an OFS”) is used as a means for further increasing the scanning speed. In the OFS, it is enough if a reflection surface of a rotating polygonal mirror has a width corresponding to a light beam width, of a wide width of an incident light beam, to substantially deflect a light beam for scanning. Therefore, it is possible to reduce a size of the rotating polygonal mirror and increase the number of surfaces thereof. Thus, the OFS is suitable for increasing the scanning speed.
However, in an image forming apparatus using the OFS, illuminance on the surface to be scanned becomes nonuniform because of features of the OFS. Therefore, there is a problem in that unevenness of a formed image in density occurs.
Hereinafter, a mechanism that the illuminance on the surface to be scanned becomes nonuniform will be described.
In the OFS, an incident light beam on a deflection means (rotating polygonal mirror) has a Gaussian distribution in which a light intensity becomes maximum near the optical axis of a condensing optical system. A reflection and deflection region is changed from the vicinity of the optical axis to peripheral portions according to a view angle. As a result, the illuminance on the surface to be scanned tends to reduce as an image height increases.
Further, in the OFS, a light beam width of a reflected and deflected light beam in a main scanning direction narrows as the view angle increases. Therefore, the tendency that the illuminance on the surface to be scanned reduces as the image height increases is further enhanced.
In particular, when an incident side F number (FNo.) in the main scanning direction is set to a small value, the light amount of peripheral part of the incident light beam on the deflection means significantly reduces. In the OFS, a different part of the incident light beam is reflected for optical scanning for each view angle, so that the nonuniformity of an illuminance distribution on the surface to be scanned is promoted.
Thus, in the OFS, in order to suppress the nonuniformity of an illuminance distribution, it is necessary to increase the incident side F number in the main scanning direction. As a result, the amount of taken light in the main scanning direction reduces. In order to ensure the required amount of light corresponding to the reduced light, the amount of taken light in the sub scanning direction needs to be increased. That is, it is necessary to set small the incident side F number in the sub scanning direction.
However, as described above, when the incident side F number in the main scanning direction is set large and the incident side F number in the sub scanning direction is set small, there arises a problem in that the size of the optical scanning device increases because an optical path length of an incident optical system is long.
Hereinafter, the reason why the optical path length of the incident optical system becomes long will be described with reference to FIGS. 10A and 10B.
FIGS. 10A and 10B are sectional views showing a principal part of general incident optical systems used for the optical scanning device. FIG. 10A is a sectional view showing the principal part in the main scanning direction (main scanning principal part sectional view). FIG. 10B is a sectional view showing the principal part in the sub scanning direction (sub scanning principal part sectional view).
In FIGS. 10A and 10B, an incident optical system 105 includes a light source means 91 and a collimator lens 104 (described later) which are integrally configured. As shown in FIGS. 10A and 10B, the collimator lens 104 for collimating a divergent light beam emitted from the light source means 91 and a sub scanning cylindrical lens 102 which is an anamorphic lens having refractive power mainly in the sub scanning direction are disposed in order from the light source means 91 side. Reference numeral 107 denotes a deflection surface.
In FIGS. 10A and 10B, a solid line indicates a light beam in the case where the incident side F number in the main scanning direction is set equal to the incident side F number in the sub scanning direction. On the contrary, a dot line indicates a light beam in the case where the incident side F number in the main scanning direction is set larger than the incident side F number in the sub scanning direction.
As is apparent from FIGS. 10A and 10B, it is necessary to lengthen an interval between the light source means 91 and the collimator lens 104 in order to obtain a desirable light beam width in the main scanning direction in accordance with the incident side F number in the main scanning direction being set large. In accordance with this, it is necessary to lengthen an interval between the sub scanning cylindrical lens 102 and the deflection surface 107. As a result, when the incident side F number in the main scanning direction is set large and the incident side F number in the sub scanning direction is set small, there arises a problem in that the optical path length of the incident optical system 105 becomes long.
In order to solve such a problem, various devices such as a multi-beam optical scanning apparatus and an optical scanning device have been proposed (for example, see Japanese Patent Application Laid-Open No. 2000-292721 and Japanese Patent Application Laid-Open No. 2001-305448).
According to Japanese Patent Application Laid-Open No. 2000-292721, a light beam enlarging optical system for enlarging a size of an incident light beam is provided on an optical path between a light source means and a deflection means. The size of the incident light beam is enlarged in at least the main scanning direction. According to Japanese Patent Application Laid-Open 2001-305448, a cylindrical lens and a collimator lens, which compose an incident optical system, are disposed in order from the light source means side, so that an optical path necessary for both is doubled to significantly shorten an optical path length.
However, in a method described in Japanese Patent Application Laid-Open No. 2000-292721, since an illuminance difference is caused between two beams at the same image height, unevenness of a formed image in density tends to occur.
Hereinafter, a mechanism to cause the illuminance difference between the two beams at the same image height in Japanese Patent Application Laid-Open No. 2000-292721 will be described with reference to FIG. 11.
FIG. 11 is a principal part sectional view in the main scanning direction (main scanning sectional view), showing a multi-beam optical scanning apparatus using an OFS described in FIG. 4 in Japanese Patent Application Laid-Open No. 2000-292721. In FIG. 11, an incident optical system is shown as a single condensing optical system 116. Reference numeral 107 denotes the deflection surface.
As shown in FIG. 11, divergent light beams emitted from the two light emitting regions 94a and 94b having an interval d1 in the main scanning direction therebetween are incident on the condensing optical system 116 and exit as parallel light beams. Assume that an intensity central light beam (located at a central position of an intensity distribution in which an intensity thereof is maximum) 94ap of the light beam emitted from the light emitting region 94a and an intensity central light beam 94bp of the light beam emitted from the light emitting region 94b are parallel to an optical axis of the condensing optical system 116.
Since the intensity central light beams 94ap and 94bp each are deviated from an optical axis 116a of the condensing optical system 116 in the main scanning direction and incident on the condensing optical system 116. Then, the intensity central light beams 94ap and 94bp are exited at an angle relative to the optical axis 116a of the condensing optical system 116 in the main scanning direction. Therefore, an interval d1′ between the intensity central light beams 94ap and 94bp of the light beams emitted from the two light emitting regions 94a and 94b on the deflection surface 107 is changed according to a distance LC between a back focal position C of the condensing optical system 116 in the main scanning direction and the deflection surface 107. The interval d1′ can be expressed byd1′=LC×d1/fcol  (1)where fcol represents the focal length of the condensing optical system 116.
According to Japanese Patent Application Laid-Open No. 2000-292721, since the distance LC between the back focal position C of the condensing optical system 116 in the main scanning direction and the deflection surface 107 is long, as is apparent from the expression (1), the interval d1′ on the deflection surface 107, between the intensity central light beam 94ap of the light beam emitted from the light emitting region 94a and the intensity central light beam 94bp of the light beam emitted from the light emitting region 94b becomes not smaller than a given value.
As a result, since the two light beams incident on the deflection surface 107 have intensity distributions La, which are nonuniform in the main scanning direction and different from each other, illuminance distributions of the two light beams at an image height on the surface to be scanned are different from each other. Such a state is shown in FIG. 12.
FIG. 12 shows illuminance distributions 94ai and 94bi of the respective light beams emitted from the two light emitting regions 94a and 94b on the surface to be scanned. As is apparent from FIG. 12, the illuminance difference is caused between two beams at the same image height on the surface to be scanned. The unevenness of a formed image in density tends to occur in Japanese Patent Application Laid-Open No. 2000-292721.
Even in the case of Japanese Patent Application Laid-Open No. 2001-305448, when the multi-beam optical scanning apparatus having the OFS is used to further increase the scanning speed, an illuminance difference is caused between a plurality of beams at the same image height on the surface to be scanned because a back focal position of the condensing optical system in the main scanning direction and a deflection surface are apart from each other.